1. Field of the Invention
The invention relates to an imaging apparatus having a low-illuminance imaging function.
2. Description of the Prior Art
Various imaging apparatuses have been proposed so far that enable shooting a subject of low illuminance.
One of such apparatus is disclosed as xe2x80x9cIMAGE PICKUP DEVICE CAPABLE OF IMAGE PICKUP UNDER LOW ILLUMINANCExe2x80x9d in Japanese patent application publication No. 05048961 (1993). FIG. 1 shows the arrangement of the image pickup device. In FIG. 1, if a low illuminance detector 90 detects a low illuminance, the gain-up controller 91 responsively increases the gains of a variable gain amplifier 92 and an AGC (automatic gain control) 93. The controller 91 further causes the signal synthesizer 94 to synthesize two time-adjacent signals or two horizontally adjacent signals for noise mitigation. Since the adjacent signal synthesis results in a noise reduction, this enables gain-controllable circuits to achieve a higher gain-up than usual.
However, synthesizing two time-adjacent signals also causes a problem of deteriorating the horizontal resolution.
The above problem is overcome by an imaging apparatus that enables low illuminance shooting by raising the gain or sensitivity through noise reduction while mitigating the deterioration in resolution in accordance with the principles of the invention.
An image signal having been imaged and amplified is converted into a series of digital pixel values. A weighted moving average of a current pixel value and pixel values of at least two pixels spatially adjacent to the pixel of the current pixel value are calculated. The weighted moving average is divided by a division factor smaller than the sum of weights used in the weighted moving average calculation. If the image signal is smaller than a predetermined value, a selector supplies the divided weighted moving average as the current pixel value to the next stage. This enables not only a gain increase due to noise reduction but also a mitigation of the deterioration in resolution
The weight of the current pixel value is preferably set substantially equal to half of the sum of the weights.
In one embodiment, the weighted moving average is calculated for the current pixel value and the pixel values of two pixels horizontally adjacent to the current pixel.
In another embodiment, the weighted moving average is calculated for the current pixel value and the pixel values of two pixels vertically adjacent to the current pixel.
In further embodiment, the weighted moving average is calculated for the current pixel value and the pixel values of four pixels horizontally and vertically adjacent to the current pixel.
In still another embodiment, the weighted moving average is calculated for the current pixel value and the pixel values of two pixels horizontally or vertically adjacent to the current. Using the divided weighted moving averages from the selector as a series of digital pixel values, the next stage first finds a second weighted moving average of the current pixel value and pixel values of two pixels linearly adjacent to the pixel of the current pixel value. In this case, the direction of the adjacent pixels is perpendicular to the adjacent pixels used in the weighted moving average calculation in the preceding stage. The second weighted moving average is divided by a division factor smaller than the sum of weights used in finding the second weighted moving average. If the image signal is smaller than a predetermined value, a second selector supplies the divided second weighted moving average as the current pixel value to the next stage.